(1) Field of the Invention
This invention relates to an antiskid type brake system for vehicles which is suitable for use in applying the brakes to a vehicle while avoiding the skidding of its wheels.
(2) Description of the Prior Art
Antiskid type brake systems are used nowadays for the purpose of applying the brakes to vehicles in as short a distance as possible without spoiling the stability of the vehicle in travel. An antiskid type brake system usually comprises brakes hydraulically actuated to check the rotation of wheels, actuators for increasing and decreasing the hydraulic pressure of the brakes, rotation sensors for monitoring the speeds of rotation of the wheels and a controller for estimating the slipping conditions of the wheels based on their speeds of rotation sensed by the rotation sensors to thereby control the actuators. The controller is usually constructed and operates such that, by assuming that the speed of rotation of the wheel of the plurality of wheels whose speed of rotation is the highest represents the vehicle speed, it estimates the vehicle speed by fixing the deceleration of the wheel whose speed of rotation is the highest as a predetermined upper limit value after the deceleration has exceeded the upper limit value so as to estimate the slipping condition of each wheel based on the difference between the estimated vehicle speed and the speed of rotation of each wheel.
It is possible to fix the upper limit value of deceleration as the highest deceleration predicted for the automotive vehicle. In actual practice, however, the lower the coefficient of friction of a road surface, the lower is the level at which the upper limit value of deceleration of a vehicle is set. In this connection, a new proposal has been made in Japanese Patent Unexamined Publication No. 26661/83 to alter the upper limit value of deceleration in accordance with the actual conditions in which the brakes are being applied. More specifically, it is proposed that the speed of rotation of a wheel which is the highest of all the speeds of rotation of a plurality of wheels of a vehicle be determined the instant the deceleration of this wheel exceeds the upper limit value of deceleration, and that the deceleration of the vehicle be estimated at a point in time when the speed of rotation of any other wheel recovers the next time, based on the sloping of a straight line connecting the speed of rotation of the wheel of the highest speed of rotation with a maximum value of the speed of rotation that recovered, so that the estimated deceleration can be used as an upper limit value the next time when it becomes necessary to fix the deceleration to the upper limit value. When this proposal is adopted, it would be possible to estimate the vehicle speed with increased accuracy and consequently to effect antiskid control of the automotive vehicle with increased precision.
However, some disadvantage are associated with this proposal. In this system, each time the deceleration of a wheel of the plurality of wheels whose speed of rotation is the highest exceeds the upper limit value and the need arises to fix the deceleration to an upper limit value, an upper limit value to which the deceleration is to be fixed when the same need arises the next time is estimated. Thus, when the need arises to fix the deceleration for the first time, an estimated deceleration has not yet been obtained. Therefore, a maximum deceleration forecast for the automotive vehicle as in the prior art would have to be used as an upper limit value for the first time. As a result, when the coefficient of friction of the road surface is so low that a high deceleration is unobtainable, an estimated vehicle speed would be at variance with the actual vehicle speed, making it impossible to effect antiskid control with a high degree of precision.